We adapted the STICS agro-environmental model to simulate the effects of cultivation practices on the biomass production and nitrogen accumulation of perennial crops undergoing regular defoliation, using alfalfa as an example. A unique set of parameters was used to simulate both establishment and regrowth phases over several years, with the assumption that crop growth is driven by interaction between crop development stage and abiotic stresses. The model accurately simulated the total biomass (stems + leaves + crown + taproot + roots) and aboveground biomass of the crop, with model efficiencies of 0.75 and 0.70, respectively, and relative root mean squared errors (rRMSE) of 42% and 36%, respectively. The evaluation results were also satisfactory with respect to total nitrogen content and the aboveground biomass nitrogen content, with model efficiencies of 0.90 and 0.60, respectively, and rRMSE values of 29% and 31%, respectively. The model thus enabled simulations of both the establishment and regrowth of alfalfa and accurately reproduced its seasonal patterns of growth, even though it tended to underestimate spring biomass production. It also produced accurate simulations of the water and nitrate contents of the soil during cropping and after crop destruction. It could therefore be a useful tool regarding the multi-criteria assessment of cropping systems based on alfalfa with respect to their sustainability.